Scattering is the change in directional distribution of radiation imparted by its passing through matter. It is usually observed using substantially monochromatic radiation, such as that from a laser or monochromator. Most of the scattered radiation is observed to have the same wavelength as that of the incident light. This directional redistribution without change in wavelength is referred to as Rayleigh scattering. A small percentage of the radiation may be scattered at a number of discrete frequencies above and below that of the incident radiation. The lines found at longer wavelengths, which are called Stokes lines, are more intense than those found at shorter wavelengths, called anti-Stokes lines. The occurrence of these discrete lines in the spectrum of the scattered light, in addition to the line of the incident light, is called the Raman effect.
Raman scattering is similar to fluorescence except for the nature of the energy-level transitions involved. Fluorescence is the emission of photons due to transitions of electrons or molecules between stable quantum energy levels after excitation by incident light. Hence, the fluorescence of a particular material always occurs at discrete wavelengths defined by the difference between that material's stable energy levels. Raman scattering is photon emission due to the transition from an unstable energy level excited by the incident light to one of the material's stable energy levels other than its initial or ground state. Hence, the Raman scattered wavelengths are functions of the differences between the material,s stable energy levels and the energy of the incident light photons, which is dependent upon the incident wavelength.
The net effect of this dependence on the wavelength of the incident light is that the wavelengths of the Raman spectrum of any given material have a predetermined difference from the wavelength of the incident light. In other words, if a Raman spectrum is measured with one wavelength of incident light, and then measured a second time using a different wavelength of incident light, the same Raman wavelength line pattern will be measured, but shifted in wavelength. The number of Raman lines and their wavelength shifts from the incident wavelength remain constant.
The wavelength shifts of the Raman lines, their intensity, and their polarization are characteristics of the scattering substance. Raman spectroscopy has proven to be very useful in characterizing the molecular content of unknown materials in, for example, the chemistry and medical testing industries.
To understand the advantages of this invention, it is helpful to first understand the basics of the current art of Raman spectroscopy. A typical prior art "general purpose" system having high sensitivity includes a polarized Argon ion laser which is focused on a sample cell. The Raman and Rayleigh scattering are collected by a lens, polarized, filtered, and focused to the entrance aperture of a double monochromator. The double monochromator uses multiple gratings to disperse the collected light into its wavelength spectrum, which is then detected by a photomultiplier tube. A continuous wavelength spectrum is scanned by mechanically rotating one or more elements in the monochromator to direct the desired wavelength path onto the photomultiplier tube.
The high-cost items in this typical package are the Argon laser, the collection lens, the double monochromator, and the photomultiplier tube. All are used because of the sensitivity required to measure Raman line intensities that are typically several orders or magnitude below that of the incident light. The Argon laser is used because of its high available power and short blue-green wavelength (the intensity of Raman scattering is proportional to the fourth power of the frequency of the incident light). The collections lens is simultaneously achromatic, large aperture, and low f/number to collect as much of the scattered light as possible. The double monochromator is used to provide a high-resolution wavelength separation with high Rayleigh and stray light rejection for low noise. The PMT is used to detect the extremely low Raman light levels present at the end of the optical path.